Recently, the number of people who use a hearing aid (shown in FIG. 12) has increased with progress of an aging society. Typically, a hearing aid is a device which amplifies ambient sound and allows a user to hear the ambient sound. However, a noise and other sounds, such as, a sound generated when a door is opened or closed (hereinafter, referred to as “a predetermined sound”), are amplified in the same manner as a voice, resulting in an excessively large output sound, since amplitude of the predetermined sound drastically increases immediately after generation of the sound. This burdens and causes the user to feel uncomfortable.
In response to the above problem, a device is suggested in which a gain is controlled by detecting a transitional noise from the outside based on a value of ratio between an effective value and an absolute value in terms of input sound which is divided into blocks for respective predetermined time zones (for example, see Patent Literature 1), as shown in FIG. 10.
Furthermore, another conventional hearing aid is suggested in which a controlled gain is decreased in a relatively short time so as to prevent the excessively large output when an input-sound pressure or an output-sound pressure increases (for example, see Patent Literature 2), as shown in FIG. 11.